The present invention relates to barium-free and lead-free ceramic glazes that can be employed with ceramic articles such as spark plug insulators.
Glazes, continuous coatings that are fusion bonded onto a substrate, can serve a variety of purposes: (1) render the substrate impermeable to liquids and gases; (2) aesthetics, e.g., covering blemishes and providing decorative effects; (3) protection; and (4) increased strength.
One property of a glaze that can be particularly important is its thermal coefficient of expansion. In order to avoid undue stresses which can cause spalling, chipping, cracking or crazing, a glaze should have a low thermal coefficient of expansion. A glaze preferably has a thermal coefficient that is similar to the ceramic substrate to which it is applied, e.g., on the order of from 6 to 7 microinches per inch per .degree. C. In fact, glazes having this low coefficient of thermal expansion can strengthen an alumina insulator by inducing compressive stresses at the surface of the glaze-insulator composite.
It is further recognized that glazes can be modified to change their properties, e.g., maturing temperature, color, and coefficient of thermal expansion. However, the highly complex, multi-component nature of glazes makes predicting the effect of varying or substituting chemical compounds in a glaze formulation difficult, even where the general properties of the individual components may be recognized.
In addition, glazes are not homogeneous, that is, they may contain one or more dispersed undissolved phases, thus the ultimate components shown by chemical analysis may not effectively describe a glaze in a manner such that the properties are readily predictable.
The composition of glazes has evolved for other reasons. For example, lead found much use in traditional glazes, however, toxicity issues surrounding lead have fostered the development of "lead-free" glazes. Examples of "lead-free" glazes can be found in U.S. Pat. No. 4,084,976 and U.S. Pat. No. 4,120,733. The composition described above includes 48 to 54% SiO.sub.2, from 7 to 11% Al.sub.2 O.sub.3, from 16.5 to 20% B.sub.2 O.sub.3, from 11 to 14% BaO, from 2 to 3% CaO, from 2 to 2.5% ZnO, from 4.25 to 5.25% Na.sub.2 O, and from 0.4 to 1% K.sub.2 O, Li.sub.2 O and MgO.
Finally, glazes that are both lead-free and barium-free are also known. See, for example, U.S. Pat. No. 4,256,497.
Spark plug insulators comprise one class of ceramic substrates that are often used with a glaze. The exterior portions of spark plug insulators are exposed to dirt and grease which may result in the formation of an electrically conducting surface and premature failure of the spark plugs. Because of this, alumina insulator bodies of spark plugs are glazed to minimize dirt and grease build-up, and to increase the strength and imperviousness of the surface.
However, the introduction of glaze onto spark plugs has encountered its own set of problems. For example, in the manufacture of a spark plug, two separate firing steps are typically employed in connection with an already sintered bisque alumina insulator. The first involves glost firing raw-glazed insulators, i.e., the ceramic component of the plug, at 2050-2150.degree. F. (1121-1177.degree. C.). A second firing step is then carried out at reduced temperatures, e.g., between 1550 and 1650.degree. F. (843 and 899.degree. C.), to incorporate a carbon-based, Fired-In Suppressor glass seal that results in the production of spark plug core assemblies, where a "core assembly" is a glazed insulator with a contained internal center electrode component.
Reduced temperatures are required in this second firing step due to the temperature-sensitive nature of certain of the components of the Fired-In Suppressor glass seal. Moreover, the necessity for carrying out two separate firing steps adds to both the cost and the time involved in glazing and glass sealing the unit.